With an ever-increasing demand for low-sulfur middle distillates, refiners have taken a keen interest in converting vacuum residuum to distillates. The search for Best Available Technology (“BAT”) has intensified over the last few years because of diminishing supplies of sweet crudes and incremental supplies coming predominantly from heavy sour crudes and heavy synthetic crudes.
Heavy crude generally refers to those crudes with high viscosity or an API gravity less than about 23. Crude oils and crude oil residuum derived from atmospheric or vacuum distillation of crude oil are examples of heavy crudes. The traditional outlet for vacuum residue was high sulfur fuel oil (“HSFO”), but HSFO demands in most regions have diminished over the last ten years giving further impetus to residue conversion processes.
One conversion technique of recent interest is resid or residuum hydrotreating. During resid hydrotreating, resid oil is upgraded with hydrogen and a hydrotreating catalyst to produce more valuable lower-boiling liquid products. Various catalytic residue-upgrading technologies are available from Chevron Lummus Global (“CLG”) including atmospheric residue desulfurization (ARDS), vacuum residue desulfurization (VRDS), up flow reactor (UFR), online catalyst replacement (OCR) and the LC-FINING®process. The LC-FINING process integrated with the ISOCRACKING® process offers a proven high conversion option. The combined process is especially attractive in situations requiring high conversion of residuum with high metals content and where diesel demand is higher than gasoline demand.
During operation of such conversion processes, foulants can form solid hydrocarbonaceous deposits on the processing equipment and associated piping, presenting numerous problems for refiners. The foulants can stick together, adhere to the sides of vessels, and agglomerate. Once entrained into any product stream, foulants are also carried away into associated downstream equipment and piping.
The situation becomes even more aggravated when two or more hydrotreating processes are connected in series as is typically done in commercial operations. In such cases, foulants not only form nucleation sites for solids growth and agglomeration in the first process, but are carried over with the hydrotreated product stream into a subsequent process where additional deposits may form.
Deposits of foulants are well known for plugging piping and tubulars, choking off pipes by reducing areas of flow, creating poor flow regimes, and interfering with the function of equipment. For example, the foulants can abrade valves and other equipment, or can build up insulative layers on heat exchanger surfaces reducing the capability to transfer heat. Continued buildup can necessitate equipment repairs, extended downtime, production shutdowns, and overall reduced efficiency and process yield.
Another aspect of foulants is that they may promote emulsions within the crude that can lead to much higher viscosities, making it difficult and challenging to pipeline the oil from one location to another. These effects are a substantial problem in heavy oil refining and transportation, and can significantly increase the costs of production to the point of removing any incentive to continue pursuit of the possible lucrative rewards of residuum conversion.
One type of foulant frequently found in heavy oil that is strongly attributable to sedimentation of deposits and high viscosity is asphaltenes. Asphaltenes are most commonly defined as a portion of crude oil that is insoluble in a low molecular weight paraffin (i.e., n-heptane, etc.), and have been found in crudes in quantities in excess of 20 percent. Asphaltenes are typically brown to black amorphous solids that are basically formed of condensed aromatic nuclei associated with alicyclic groups. In addition to carbon and hydrogen, the complex atomic structure can also include nitrogen, oxygen, and sulphur atoms. Particle size can range less than 0.03 microns to several thousand microns, and can be characterized as sticky or cohesive, and may agglomerate.
Asphaltenes are polar molecules which aggregate together through aromatic π-π orbital association, hydrogen bonding, and acid-base interactions. They exist in the form of colloidal dispersions stabilized into thermodynamic equilibrium by other components in the crude oil. However, the equilibrium of the oil can be disrupted during a production process, or any other mechanical or physicochemical processing where changes in pressure, temperature and phase composition may occur. This destabilizes the asphaltene, leading to aggregation and deposition of the particles into the surroundings.
Many processes beneficial in the production of crude are limited because the processes also provide conditions beneficial to the formation of deposits. Various methods have been used to clean and prevent deposit formation, as well as to reduce viscosity of the heavy crudes. In one method, deposits are controlled by stringently controlling surrounding conditions. In U.S. Pat. No. 4,381,987, a hydrocarbon feedstream containing asphaltenes is hydroprocessed by passing the stream through a catalytic reaction zone in the presence of a catalyst bed. It is disclosed therein that plugging of the catalyst bed can be avoided by controlling the severity of the hydroprocessing conditions in the catalytic reaction, decreasing the likelihood of asphaltenes forming deposits. However, the environment outside of the reactor zone is not as predictable, and comparable control outside of the zone is unobtainable.
In U.S. Pat. No. 5,139,088, asphaltene precipitation in the flow path of an oil production well is claimed to be inhibited by injecting a heavy fraction of crude oil having a relatively high aromaticity and molar weight.
In U.S. Pat. No. 4,081,360, issued Mar. 28, 1978 to Tan et al., a light solvent is added to coal liquefaction fractions for suppressing the formation of asphaltenes.
A variety of chemical treatments are also disclosed in the art for affecting foulants including the use of dispersants and viscosity reducing agents. The dispersant-plus-solvent approach has been disclosed for affecting asphaltenes, and a variety of suitable dispersant compositions are known and available to the trade for this purpose, such as disclosed by U.S. Publication 2006/0014654. Asphaltene precipitation inhibitors have also been disclosed for use in continuous treatment or squeeze treatments of well formations.
However, feed sources can vary significantly in their composition, and individual dispersing agents and viscosity reducing agents can operate effectively only in a limited range. Even small changes in the oil composition can have a major effect on the dispersing properties for asphaltenes. Also, even though dispersants and precipitation inhibitors address the problem of slowing or preventing asphaltene precipitation, once deposits form, the use of such inhibitors is negated because the removal generally requires a cleaning, scraping or hydrotreating procedure to remove the deposits. This is undesirable as it usually requires a reduction or complete shut-down of production.